Cheap charcoal filters could greatly improve in-car air quality

A cheap charcoal air filter can reduce nitrogen dioxide (NO2) inside vehicles by as much as 90 per cent, compared to levels outside the vehicle, according to researchers.

In a report by WM Air, the West Midlands Air Quality Improvement Programme at the University of Birmingham, the research team demonstrated that charcoal filters, which typically cost around £10-£20, can effectively remove NO2 from the air within vehicle cabins.

NO2 is a common air pollutant that can aggravate diseases such as asthma and increase the risks of respiratory infections. Traffic emissions are a dominant source of NO2, so road users inside vehicles are exposed as air circulates into vehicle cabins from outside through open windows and ventilation systems.

While ventilation systems do currently filter air, this is typically via a pollen filter. These prevent tiny particles and pollen getting inside the vehicle, but they have little effect on gases such as NO2. The activated carbon filters, in contrast, remove NO2 through a process called adsorption, in which the NO2 reacts with the carbon to stick to the surface area of the filter.

As with pollen filters, the effectiveness of the carbon filter will decrease over time, meaning it would need to be replaced regularly when the vehicle is serviced.

Dr Vasileios Matthaios, lead researcher, said: “Our findings show clearly that there are benefits to switching to activated carbon air filters, reducing exposure to NO2 and the risk of related adverse health effects. These filters are simple, effective and inexpensive and should be considered, particularly for people who spend long periods of time in vehicles, such as professional drivers.”

In the study, the researchers tested NO2 in 10 different vehicles, ranging in size and type (petrol, diesel, hybrid and electric vehicles were all included). Air quality measurements inside the vehicles were taken with a range of ventilation conditions (AC turned on or off and windows either closed or partially open).

Each vehicle was tested three times: firstly, with its original air filter in place; secondly, with a pollen filter fitted; lastly, with the activated charcoal filter fitted.

The researchers found that, overall, in-vehicle NO2 concentrations were on average 1.6 times lower when the windows were closed and the ventilation system recirculated air, compared to levels when the windows were open. When new standard pollen filters were fitted, NO2 concentrations were almost unchanged between closed windows and fresh air coming through the ventilation system and with windows open.

However, with activated carbon filters fitted, in-vehicle NO2 levels were on average 14.3 times lower with closed windows and recirculated air. Even with fresh air coming through the ventilation system, NO2 levels were 6.6 times lower than levels with the windows open. Maintaining appropriate ventilation in a car is important to prevent drowsiness.

Professor William Bloss, co-author on the paper, said: “These results show a fairly simple way to improve air quality inside vehicles, although as the main source of NO2 in our cities is diesel vehicles, reducing traffic emissions overall will bring the greatest air quality benefit across the general population.”

The research paper – ‘NO2 levels inside vehicle cabins with pollen and activated carbon filters: A real world targeted intervention to estimate NO2 exposure reduction potential‘ – outlining the team’s findings has been published in the journal Science of the Total Environment.

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